Common welding methods

I. What is brazing? How is brazing classified? What are the characteristics of brazed joint forms? Brass welding involves using a metal with a lower melting point than the base material as a filler metal. When heated, the filler metal melts while the base material does not; the liquid filler metal then wets the base material, fills the gaps between the parts, and diffuses with the base material, thereby firmly joining the parts together. Based on the melting point of the filler metal, brazing is divided into soft brazing and hard brazing. (1) Soft soldering: The solder used in soft soldering has a melting point below 450°C, and the strength of the joint is low (less than 70 MPa). (2) Hard brazing: The filler metal used in hard brazing has a melting point above 450°C, and the strength of the joint is high (greater than 200 MPa). The load-bearing capacity of a brazed joint is related to the size of the joint interface. Therefore, lap joints and socket joints are generally used in brazing to compensate for the insufficient strength of brazing. II. What are the classifications of arc welding, and what are its advantages? The welding method that uses an arc as a heat source is called arc welding. It can be divided into three types: manual arc welding, submerged arc automatic welding, and gas shielded welding. The greatest advantage of manual automatic welding is its simple equipment, flexible and convenient application, wide range of applicability, and the ability to weld in various positions as well as straight seams, circumferential seams, and various types of curved seams. It is particularly suitable for applications where the operating conditions remain unchanged and for welding short welds ； Submerged arc automatic welding features high productivity, good weld quality, and favorable working conditions ； Gas shielded welding features good protection, stable arc, and concentrated heat. III. During shielded metal arc welding, what are the characteristics of the composition of the welded joints in low-carbon steel, as well as the microstructure and properties of the metal in various regions? (1) A welded joint consists of weld metal and the heat-affected zone. 1) Weld metal: During welding heating, the temperature at the weld area is above the liquidus line; the base material and the filler metal form a common molten pool, which upon cooling develops a cast structure. During the cooling process, the liquid metal crystallizes from the fusion zone toward the center of the weld, forming a columnar crystal structure. Due to the alloying effect of the electrode core and coating during welding, the chemical composition of the weld metal is usually superior to that of the base material. As long as the electrodes and welding process parameters are chosen appropriately, the strength of the weld metal is generally not lower than that of the base material. 2) Heat-affected zone: The area on either side of the weld where the metal undergoes changes in structure and properties as a result of the heat generated during welding. (2) The heat-affected zone of low-carbon steel is divided into the fusion zone, the overheated zone, the normalized zone, and the partial transformation zone. 1) The fusion zone is located between the weld and the base metal; some of the metal is melted while some remains unmelted, and it is also referred to as the semi-melted zone. The heating temperature is around 1,490–1,530°C; in this range, the composition and microstructure are highly uneven, strength decreases, and plasticity is very poor. This is where cracks and localized brittle failure occur. 2) Superheating zone: Located right next to the fusion zone, with a heating temperature of approximately 1,100–1,490°C. As the temperature **exceeds Ac3, the austenite grains grow rapidly, resulting in a superheated microstructure that reduces plasticity**, with the impact toughness value decreasing by approximately 25% to 75%. 3) Normalizing zone: The heating temperature is approximately 850–1,100°C, which falls within the normal range for normalizing heating. After cooling, a uniform fine ferrite and pearlite structure is obtained, whose mechanical properties are superior to those of the base material. 4) In some phase transition regions, the heating temperature is approximately 727–850°C. Only some of the tissues undergo transformation; after cooling, the tissue structure is uneven and its mechanical properties are poor. IV. What is resistance welding? What are the different types of resistance welding, and in what applications are they used? Resistance welding is a welding method that utilizes the resistive heat generated by electric current flowing between the workpieces and their welding contact surfaces to heat the materials to a plastic state or to a state of partial melting, after which pressure is applied to form a welded joint. Resistance welding is divided into three types: spot welding, seam welding, and butt welding. (1) Spot welding: The workpieces are pressed between two cylindrical electrodes; electricity is applied to heat them, causing the material at the contact points to melt and form a molten core. After the power is turned off, the material solidifies under pressure, resulting in a weld joint with a dense structure. Spot welding is suitable for welding thin sheets (with lap joints) of 4 mm or less as well as steel rebar, and is widely used in the production of automobiles, aircraft, electronics, instruments, and everyday items. (2) Seam welding: Seam welding is similar to spot welding, with the difference being that a rotating disc-shaped electrode is used instead of a cylindrical electrode. The stacked workpieces are pressed and energized between the disks, and fed forward as the disks rotate, thus forming a continuous weld seam. Seam welding is suitable for welding thin sheet overlaps with a thickness of less than 3 mm, and is mainly used in the production of sealed containers and pipes. (3) Butt welding: Depending on the welding process, butt welding can be divided into resistance butt welding and flash butt welding. 1) Resistance butt welding: In this welding process, a upsetting force of 10–15 MPa is first applied to bring the joint surfaces of the workpieces into close contact. The area is then heated electrically until it reaches a plastic state. After that, an even higher upsetting force of 30–50 MPa is applied, while the power is turned off; this pressure causes plastic deformation at the joint site, thereby enabling welding. Resistance welding is simple to operate and produces joints with a smooth surface, but it requires high precision in the machining and cleaning of the end faces of the parts to be welded; otherwise, uneven heating of the contact surfaces can lead to defects such as oxide inclusions and incomplete welding, thereby affecting the quality of the weld. Therefore, resistance welding is generally used only for welding workpieces with a diameter of less than 20 mm, simple cross-sections, and low stress levels. 2) Flash butt welding: During this welding process, electricity is applied first, after which the two workpieces are brought into light contact. Due to the uneven surface of the workpieces, the current density at the point of contact is very high, causing the metal to melt, vaporize, and explode rapidly; sparks are generated as a result, giving rise to the flash phenomenon. Continue moving the weld pieces to create new contact points; the flashing phenomenon will occur repeatedly. Once both end surfaces of the weld pieces have been fully melted, apply pressure quickly, then turn off the power and continue applying pressure to fuse the weld pieces together. Flash butt welding produces joints of high quality, and it requires little pre-welding cleaning of the joint surface. It is commonly used for welding important components that are subject to high stresses. Flash butt welding can be used to weld not only the same type of metals but also dissimilar metals such as aluminum and steel, or aluminum and copper. It can be used to weld metal wires as thin as 0.01 mm, as well as pipes with a diameter of 500 mm and sheets with a cross-sectional area of 20,000 mm2. V. What is the basic principle of laser welding? What are its features and uses? Laser welding uses a focused laser beam as a source of energy; the heat generated by this beam is used to weld the workpiece. Laser welding has the following characteristics: 1) The laser beam has a high energy density, resulting in an extremely short heating time; the weld spot is small, the heat-affected zone is narrow, welding deformation is minimal, and the dimensional accuracy of the welded parts is high ； 2) It can weld materials that are difficult to weld using conventional welding methods, such as refractory metals like tungsten, molybdenum, tantalum, and zirconium ； 3) Non-ferrous metals can be welded in air without the need for an additional shielding gas ； 4) Laser welding equipment is complex and costly. Laser welding can be used to weld low-alloy high-strength steels, stainless steels, as well as copper, nickel, titanium alloys, and more ； Allometallic materials as well as non-metallic materials (such as ceramics, plexiglass, etc.) ； It is currently mainly used in fields such as electronic instruments, aviation, aerospace, and nuclear reactors. VI. What is the basic principle of electron beam welding? What are its features and uses? Electron beam welding uses a focused high-speed electron beam in a vacuum to bombard the welding surface, causing it to melt instantly and form a weld joint. Electron beam welding has the following characteristics: 1) High energy density and strong electron penetration ； 2) Fast welding speed, no heat affect, minimal welding deformation ； 3) It provides good vacuum protection and results in high-quality welds, making it particularly suitable for welding reactive metals. Electron beam welding is used to weld low-alloy steel, non-ferrous metals, refractory metals, composite materials, dissimilar materials, etc., and it can be applied to both thin and thick plates. It is particularly suitable for welding thick parts and components that require minimal deformation, devices used in vacuum environments, as well as precision micro-components.

Common welding methods are divided into three categories: fusion welding, pressure welding, and brazing. Welding methods include carbon dioxide gas shielded welding, manual arc welding, submerged arc automatic welding, electroslag welding, tungsten inert gas welding, and plasma arc welding. Carbon dioxide gas shielded welding can effectively prevent the harmful effects of air on the molten metal. It can be used for low-carbon steel and low-alloy steel.